An aircraft is required to perform many different operational modes in a single flight cycle. Such modes comprise taxiing, take-off, climb, cruise, holding, decent and landing to which the engines of the aircraft must also comply with related performance requirements.
Conventional gas turbine engines, particularly turbofans, operate at relatively low exhaust nozzle pressure ratios, where it is sufficient to use a convergent nozzle to develop thrust, as the pressure loss due to external expansion of the working exhaust gases is relatively low. With increasing flight speed the nozzle pressure ratio increases, and the expansion of the gases needs to be controlled to reduce losses, normally using a convergent-divergent nozzle. In the ideal setting at cruise, the nozzle exit pressure is reduced to ambient by the divergent part of the nozzle. If the nozzle comprises fixed convergent-divergent geometry, optimised for cruise, the losses at lower nozzle pressure ratios would increase due to either over expansion of the exhaust gases or exhaust flow separation. Thus it is preferable for the area ratio of the convergent-divergent nozzle to be varied with flight speed and nozzle pressure ratio.
It is well known to vary the area of an exhaust nozzle; one such application is the Olympus™ engines on Concorde™ aircraft as well as the propulsion systems on most of the World's military fighter aircraft. However, these are generally complex multi-variable devices, typically consisting of systems of overlapping petals and comprising numerous moving components and multiple actuation systems.
U.S. Pat. No. 4,527,388 granted to The Garrett Corp., discloses a turbofan engine comprising a core engine and a fan bypass passage, both discharging streams of pressurised gases through a mixing section and into an exhaust nozzle. The exhaust nozzle defines a throat through which the combined gas streams are accelerated and discharged therethrough. The exhaust nozzle also comprises a single axially moveable and rearwardly tapering centre-body, which is moveable relative to the remainder of the exhaust nozzle to simultaneously vary both the throat area and one of the respective fluid flow areas through which the core engine and fan bypass passage fluid streams flow. When the engine produces high power the centre-body is translated rearward, for instance at takeoff, such that the areas of the exhaust nozzle and core are both minimum. This increases the velocity of the gas stream and intrinsically disadvantageously increases the amounts of exhaust or jet noise. Furthermore, at a lower engine power, for instance relatively slow cruise; the centre-body is in its forward-most position where the mixing section is generally divergent in area. This cruise position leads to a reduced gas stream velocity relative to the ambient air and disadvantageously provides a less thrust-capable engine.